[LeetCode] 123. Best Time to Buy and Sell Stock III

Say you have an array for which the ith element is the price of a given stock on day i.
Design an algorithm to find the maximum profit. You may complete at most two transactions.
Note:
You may not engage in multiple transactions at the same time (ie, you must sell the stock before you buy again).

Thought process:
Iterate through the prices. Maintain variables to keep track of total profit at first buy, first sell, second buy and second sell.
  • First buy: lowest price in prices.
  • First sell: Highest profit after selling first stock.
  • Second buy: Highest profit after buying second stock.
  • Second sell: Highest profit after two transactions.

Solution:

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class Solution {
    public int maxProfit(int[] prices) {
        int firstBuy = Integer.MAX_VALUE;
        int firstSell = 0;
        int secondBuy = Integer.MIN_VALUE;
        int secondSell = 0;
        
        for (int price : prices) {
            firstBuy = Math.min(firstBuy, price);
            firstSell = Math.max(firstSell, price - firstBuy);
            secondBuy = Math.max(secondBuy, firstSell - price);
            secondSell = Math.max(secondSell, secondBuy + price);
        }
        return secondSell;
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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